1. Field of the Invention
The invention relates to an injection compression molding method, and also to a toggle type mold clamping apparatus, further relates to a method of controlling a flow rate of a mold clamping cylinder of a toggle type injection molding machine and still further relates to a toggle type mold clamping apparatus, and particularly to a toggle type mold clamping apparatus having an overload protecting function.
2. Related Art
An example of a prior art injection compression molding apparatus is shown in Unexamined Japanese Patent Publication No. SHO 61-253520. The injection compression molding apparatus is of the so-called direct acting type. In the apparatus, a movable platen is directly attached to a piston rod of a mold clamping cylinder so that the movable platen is moveable the same speed (by the same moving distance) as that of the piston rod. In one of known injection compression molding methods, molten resin is injected into a cavity at a given mold closing stroke position at which the molds are not completely closed and the compression process is started at the middle of the mold closing process. That is, a relatively long compression stroke is set so that molten resin is sufficiently compressed. This method is frequently employed in a foam molding or the like.
However, such a direct acting type injection compression molding apparatus of the prior art has a problem in that it is cumbersome to control the position of the movable platen. In an injection compression molding apparatus, there may be a case where, in a compression step wherein molten resin supplied into the mold cavity is subjected to a process of reducing the capacity of the cavity, a compression force must be exerted in a stepwise manner by advancing the movable platen in stages, or a case where a step of slightly increasing the cavity capacity (a strain relieving step) must be conducted in the course of the molding process in order to relieve strain of a molded piece due to the compression molding. In order to conduct such a positional control on the movable platen, therefore, a high-precision sensor for accurately detecting the position of the movable platen, and a high-precision flow rate control device for stopping the movable platen at a given position are required. This makes the positional control of the movable platen cumbersome.
Such a direct acting type injection compression molding apparatus of the prior art has another problem in that it is cumbersome to control the speed of the movable platen. When a molding process is conducted using a long compression stroke as described above. Particularly, the speed of the movable platen must be controlled over the long stroke. In an injection compression molding, a compression force is applied to molten resin supplied into the mold cavity by reducing the cavity capacity (or advancing the movable platen), and therefore the control of the speed of the movable platen must be started from the beginning of the compression process. In the case of a direct acting type mold clamping apparatus, furthermore, a final compression step is conducted under a state where the fixed mold is butted against the movable mold (the mold-contacting state). In order to prevent the molds from being damaged by shock due to the butting, the deceleration of the movable platen must be started from a position which is in front of the butting position, thereby making the control of the speed of the movable platen cumbersome.